> The question to ask is: "Is it even physiologically possible for a small,>hairless hominid to be a ubiquitous-wader/occassional-swimmer, and not have>problems with hypothermia?"

No, or yes.

It would depend on the temperature of the water.

>If even a FAT hairless primate of Lucy's mass could not be habitually a>wader/swimmer without suffering from hypothermic effects (even in warm>water), then the whole theory is essentially demolished.

We'd have to know the temperature around the area where Lucy
and descendants lived. If the air was dry then it would get
cold at night. Why then would animals be losing their fur.
Secondly, ceteris paribus, if the air was dry in all likelihood
there would have been less water in the oceans. If there had been
more water, more of it would have evaporated, (and since this
water would have to come from the ice caps) and would imply that
the earth was warmer. In both cases, the water (ocean) temperature
would have stayed constant. In the humid case, it would stay
warm at night too.

How hot is the water temperature around the equator now, and how
hot would it have been then?

So the water could have been useful either to keep them warm
or cool. How could they have kept warm at night if it was
dry weather and why would they be losing their fur around this
time when it is getting colder (or was already colder).